Civil Engineering Reference
In-Depth Information
Blast Resistance
• Resistant to attack by acid (farmyard) wastes
• Price effective
The mix used had a water to portland cement ratio of
0.33 and had a 7-day compressive strength of 50 MPa
(7,250 psi).
High-performance concrete can be designed to have excel-
lent blast resistance properties. These concretes often have
a compressive strength exceeding 120 MPa (14,500 psi)
and contain steel fibers. Blast-resistant concretes are often
used in bank vaults and military applications.
Diffusion
Permeability
Aggressive ions, such as chloride, in contact with the
surface of concrete will diffuse through the concrete until
a state of equilibrium in ion concentration is achieved. If
the concentration of ions at the surface is high, diffusion
may result in corrosion-inducing concentrations at the
level of the reinforcement.
The lower the water-cementing materials ratio the
lower the diffusion coefficient will be for any given set of
materials. Supplementary cementing materials, particu-
larly silica fume, further reduce the diffusion coefficient.
Typical values for diffusion for HPC are as follows:
Type of Concrete
The durability and service life of concrete exposed to
weather is related to the permeability of the cover concrete
protecting the reinforcement. HPC typically has very low
permeability to air, water, and chloride ions. Low perme-
ability is often specified through the use of a coulomb
value, such as a maximum of 1000 coulombs.
Test results obtained on specimens from a concrete
column specified to be 70 MPa (10,000 psi) at 91 days and
which had not been subjected to any wet curing were as
follows (
Bickley and others
):
Water permeability of vacuum-saturated specimens:
Age at test: 7 years
Applied water pressure: 0.69 MPa
Permeability: 7.6 x 10
-13
cm/s
Rapid chloride permeability (ASTM C 1202):
Age at test, years Coulombs
1 303
2 258
7 417
The dense pore structure of high-performance con-
crete, which makes it so impermeable, gives it charac-
teristics that make it eminently suitable for uses where a
high quality concrete would not normally be considered.
Latex-modified HPC is able to achieve these same low
levels of permeability at normal strength levels without
the use of supplementary cementing materials.
A large amount of concrete is used in farm structures.
It typically is of low quality and often porous and with a
rough surface, either when placed or after attack by farm-
yard wastes.
Gagne, Chagnon, and Parizeau (1994)
provided a case
history of the successful application of high performance
concrete for agricultural purposes. In one case a farmer
raising pigs on a large scale was losing about 1 kg per pig
through diarrhea. This problem was resolved by recon-
structing the pig pens with high performance concrete.
Cited as beneficial properties in this application were:
• Surface smoothness that is compatible with the sensi-
tive skin of a piglet
• Non-slip surface
• Good thermal conductivity resulting in uniform
distribution of heat
• Impermeable surface to resist the growth of bacteria
and viruses
• Easy to place
Diffusion Coefficient
Portland cement-fly-ash
silica fume mix:
1000 x 10
-15
m
2
/s
Portland cement-fly ash mix:
1600 x 10
-15
m
2
/s
Carbonation
HPC has a very good resistance to carbonation due to its
low permeability. It was determined that after 17 years the
concrete in the CN Tower in Toronto had carbonated to an
average depth of 6 mm (0.24 in.) (
Bickley, Sarkar, and
Langlois 1992
). The concrete mixture in the CN Tower had
a water-cement ratio of 0.42. For a cover to the reinforce-
ment of 35 mm (1.4 in.), this concrete would provide
corrosion protection for 500 years. For the lower water-
cementing materials ratios common to HPC, significantly
longer times to corrosion would result, assuming a crack
free structure. In practical terms, uncracked HPC cover
concrete is immune to carbonation to a depth that would
cause corrosion.
Temperature Control
The quality, strength, and durability of HPC is highly
dependent on its temperature history from the time of
delivery to the completion of curing. In principle, favor-
able construction and placing methods will enable: (1) a
low temperature at the time of delivery; (2) the smallest
possible maximum temperature after placing; (3) mini-
mum temperature gradients after placing; and (4) a
gradual reduction to ambient temperature after maximum
temperature is reached. Excessively high temperatures
and gradients can cause excessively fast hydration and
micro- and macro-cracking of the concrete.
It has been a practice on major high-rise structures
incorporating concretes with specified strengths of 70 MPa
